1. Field of the Invention
This invention relates to method and apparatus for manufacture of a continuous sheet of flat glass by supporting molten glass in a pool of molten metal and advancing it along the surface of the pool of molten metal while cooling it to form a continuous sheet of flat glass. More particularly, this invention relates to a temperature control system for regulating the flow of tin within the molten metal bath on which the glass is formed.
2. Brief Description of the Prior Art
It has been recognized in the Prior Art that the forming chamber containing the molten tin bath acts to remove heat from the molten glass and cause it to solidify. Various temperature regulation means for the forming chamber have been suggested.
U.S. Pat. No. 3,733,190 to Lawrenson discloses thermal regulators in contact with the molten metal bath.
In U.S. Pat. No. 3,241,937 to Michalik et al a bath containing cooling elements embedded in the refractory bottom of the bath is disclosed.
In U.S. Pat. No. 3,409,423 to deLajarte it is disclosed that cooling units may be installed transverse of the tank holding the molten metal bath.
U.S. Pat. No. 3,930,829, a modification of the float glass process wherein the glass is lifted vertically from the bath, discloses a short bath chamber in which cooling members are embedded within the bottom refractory of the float glass forming chamber.
In U.S. Pat. No. 3,223,503 to Barradell-Smith et al a process for regulating the path of the glass ribbon in a float chamber by selective cooling is disclosed. The cooling is carried out at the surface of the bath near the ribbon edge to create currents that will shift the path of the glass ribbon.
As can be seen from the above references the regulation of the temperature of the molten metal bath in a forming chamber by cooling members is known. However, while temperature regulation by both cooling and heating means is known and has led to improved glass they have not served to overcome the problem of longitudinal defects caused by uneven thickness of the glass that results because of uneven temperatures transversely across the glass during the ribbon forming of the float formation process. The methods of the prior art all use symmetrical cooling either as temperature regulating means in the molten metal bath or embedded in the bottom of the bath. The cooling or other heat regulation means form convection currents within the molten metal that forms the bath. These currents are symmetrical on either side of the center line of the forming chamber. These convection currents created by the symmetrical temperature regulating means in combination with the molten metal current created by the glass movement tends to contribute to longitudinal defects of the glass as the currents within the molten metal tend to produce a temperature effect at the middle of the molten metal bath that is different from that at the edges of the bath. Therefore, there is a need for a float glass formation process that will eliminate the difficulty of a temperature gradient across the transverse direction of the glass as it moves through the forming chamber. There is a need to eliminate temperature gradients of the glass ribbon in the transverse direction caused by the current in the molten metal that is closely adhered to the bottom of the glass during its movement in the bath.